1. Field of the Invention
This invention relates to an image forming apparatus in which a toner image on an image bearing member is repetitively transferred to a transferring medium, wherein the lowering of the quality of image when the toner image is transferred to the transferring medium is prevented.
2. Related Background Art
When a toner image on an image bearing member is to be repetitively transferred to a transferring medium, as shown in FIG. 18 of the accompanying drawings, there is the problem that there occurs so-called “re-transferring” in which when a toner image transferred from an image bearing member 1a on the upstream side with respect to the direction of movement (arrow R51) of a transferring medium 51 to the transferring medium 51 is transferred to an image bearing member 1b on the downstream side when a toner image on the image bearing member 1b is transferred to the transferring medium 51. Due to the occurrence of the re-transferring, an image of desired density is not obtained and the quality of image is lowered.
Heretofore, in order to solve the problem of this re-transferring, as shown in FIG. 19 of the accompanying drawings, a transfer member 5 for transferring a toner image on an image bearing member 1 to a transferring, medium 51 has been provided upstream of an area N in which the image bearing member 1 and the transferring medium 51 contact with each other, with respect to the direction of movement (R51) of the transferring medium 51.
The cause of the occurrence of the re-transferring is considered to be that the charging polarity of the toner image on the transferring medium 51 is reversed by an electric current flowing through the area N in which the image bearing member 1 and the transferring medium 51 contact with each other. The more becomes the electric current flowing through the area N in which the image bearing member 1 and the transferring medium 51 contact with each other, the greater becomes the amount of toner of which the charging polarity is reversed, and the re-transferring becomes liable to occur.
If the transfer member 5 is provided upstream of the area N in which the image bearing member 1 and the transferring medium 51 contact with each other with respect to the direction of movement (R51) of the recording medium, discharge occurs at a gap A upstream of the area N in which the image bearing member 1 and the transferring medium 51 contact with each other. By this discharge, the potential difference between the surface of the image bearing member 1 and the surface of the transferring medium 51 becomes small. Thereupon, the electric current flowing through the area N in which the image bearing member and the transferring medium contact with each other during transferring becomes little, and the occurrence of the re-transferring is prevented.
However, if the transfer member 5 is provided upstream of the area N in which the image bearing member 1 and the transferring medium 51 contact with each other with respect to the direction of movement of the transferring medium 51, so-called “scattering” in which the toner scatters around the toner image transferred to the transferring medium 51 occurs. The quality of image is lowered by the occurrence of the scattering. The cause of this scattering is considered to be that due to the discharge at the gap A, the toner image on the image bearing member flies to the recording medium at the gap A.
Here, the re-transferring and the scattering cannot be prevented, and there has arisen the problem that the quality of image is lowered.